Plastic tubing electron irradiation apparatus with roller means to flatten the tubing during irradiation

ABSTRACT

A process and apparatus for irradiating plastic tubing such that the dosage imparted to the tubing is uniform as to depth and area. The tubing is passed to a flattening zone, then to an irradiation zone, then reversed, inverted, and the sequence is repeated a plurality of times. The flattening zone comprises a flat roller or equivalent means. The irradiation zone comprises an electron beam gun or an equivalent irradiation source. The reversing and inverting is accomplished by a series of wheels and guide rollers arranged adjacent to the flat rollers.

iJnited States' Patent Spillers Apr. 10, 1972 54] PLASTIC TUBINGELECTRON 3,027,312 3/1962 Lanning ..250/49.5

FOREIGN PATENTS OR APPLICATlONS TUBING DURING IRRADIATION 817,033 7/1959Great Britain ..250/49.s

Inventor: Frank W. Spillers, Lake Jackson, Tex.

The Dow Chemical Company, Midland, Mich.

Filed: July 14, 1969 Appl. No.: 859,604

Related U.S. Application Data Division of Ser. No. 631,793, Apr. 18,1967, abandoned.

Assignee:

U.S. Cl. .....250/49.5 TE, 250/52 Int. Cl. ..H0lj 37/00 Field of Search..250/49.56, 49.57, 52

References Cited UNITED STATES PATENTS 1/1961 McDonald ..250/49.5

M0 /or o r/v en Primary Examiner-James W. Lawrence Assistant Examiner-C.E. Church Attorney-Griswold & Burdick, Lester J. Dankert and B. G.Coll'ey [5 7] ABSTRACT A process and apparatus for irradiating plastictubing such that the dosage imparted to the tubing is uniform as todepth and area. The tubing is passed to a flattening zone, then to anirradiation zone, then reversed, inverted, and the sequence is repeateda plurality of times. The flattening zone comprises a flat roller orequivalent means. The irradiation zone comprises an electron beam gun oran equivalent irradiation source. The reversing and inverting isaccomplished by a series of wheels and guide rollers arranged adjacentto the flat rollers.

4 Claims, 4 Drawing Figures cubic/e PATENTEDAPR 18 I972 SHEET 10F 2K693. Q00 Q O OQ INVENTOR. Frank MA 5 0/'//e/"5 HTTURNEYS PATENTEDAPR 18m2 35571533 SHEET REF 2 INVENTOR. Frank W s ofl/efis HTTOR/VfYSBACKGROUND This invention relates to a process of and apparatus for theuniform irradiation of plastic tubing, such as tubing of polyethyleneand copolymers of ethylene.

It is known that in order to prepare bilaterally oriented films ofpolymers of ethylene (heat shrink films) one can first melt and extrudea relatively thick (4-60 mils) continuous tube of the plastic, cool it,irradiate it with electrons, X-rays, gamma rays, beta rays or ultraviolet light, and then heat and stretch it lengthwise and laterally (US.Pat. No. 3,126,680). It has been a problem of long standing in this artto provide a method and/or apparatus which will provide a uniform dosageof irradiation to the tubing. One suggested attempt to provide asolution to this problem has been the use of balls or cylinders ofradioactive material within the tubing in conjunction with an externalsource of radiation (US. Pat. No. 3,027,312). Another suggested attemptto provide a solution has been the use of a main cathode ray generatorworking in conjunction with an auxiliary generator (US. Pat. No.2,724,059). Another suggested technique has been to use multiple passesof the tubing under an electron beam gun having a circular pattern toensure uniform dosage (US. Pat. No. 2,914,450).

SUMMARY It has been observed that even with an electron beam gun havinga uniform transverse pattern (see US. Pat. No. 2,602,751 the irradiationof tubing passing through the beam is not uniform by the processes ofthe prior art or else the cost of the equipment for the uniform dosageprocesses are too high to warrant use in high volume production runs ofthe desired heat shrink film.

More specifically, when a plastic tubing is irradiated by a singleelectron beam generator by passing it a plurality of times under thebeam, the creaseline of the flattened tubing (which is where the tubefolds back on itself) does not receive a uniform dosage of irradiationbecause ionization is not uniform in depth. This results in areas oflittle or no cross linking which when the tube is biaxially orientedproduces corresponding areas of thin spots or non-uniformity in thefilm. This is highly undesirable since the shrink film is used topackage'meat products to a large extent. These products are subject to.rough handling in the market place and these weak spots or. thin spotsare a potential source of punctures and the subsequent contaminationand/or spoilage of the product.

It has been observed that when the tubing is passed over two spacedrolls and under an electron beam gun located between said rolls as inthe prior art, the tubing does not remain flat but assumes an ellipticalcross section due to its inherent resiliency and the pressure of thehydrogen gas which is generated inside the tubing during the irradiationoperation. This invariably results in areas of low dosage at the creaseline of the tubing since the ionization efiiciency of an electron beamvaries widely from the distance to the generator. Thus, 60 percent ofthe maximum ionization occurs at the surface of a substrate, 100 percentionization occurs at one-third the total depth of penetration, andlittle or no ionization occurs at the maximum depth of penetration.

Now, in accordance with this invention, it has been found that if thetubing is completely flattened in the zone of irradiation asubstantially uniform dosage of ionization is, achieved.

' This flattening is accomplished by passing the tubing over a flatroller directly under the electron beam generator. An alternate methodand/or apparatus to accomplish the desired flattening, is to use aplurality of pairs of vertically spaced flat plates arrange directlyunder the electron beam generator so as to receive the irradiation.These plates are spaced a passes therethrough, it is completelyflattened. These plates are somewhat, wider than the flattened tubingand have an opening or a series of openings in the upper plate as wideas the width of the flattened tubing so as to permit full and directirradiation of the tubing as it passes, through the irradiation zone inaflattenedstate. A further alternate method to accomplish the desiredflattening is to create a vacuum inside the plastic tubing bycompressing it against a stationary plate by meansof a pair of idlerollers mounted on opposite ends of an arm rotating about a poweredshaft at the middle of the arm in a direction opposite to the feed ofthe tubing. This is accom plished just prior to the irradiation. It ispreferred to use the flat rollers for the flattening step since this ismore economical. For the purposes of this invention, the terms flattenedstate or flattened means that the top section of the tubing is in actualcontact with the lower half. Thus, the flattening rollers are consideredto be the primary rollers and the secondary rollers are the associatedrollers which reverse and direct the tubing through the flattening andirradiation zone the required or desired number of times to achieve thedesired uniform irradiation or ionization.

DESCRIPTION OF THE DRAWING The features and advantages of this inventionmay be best understood by reference to the accompanying drawings.

FIG. 1 shows a schematic diagram of the overall process and apparatus.

FIG. 2 shows an expanded isometric view of the details of the conveyorwithin the radiation cubicle.

FIG. 3 shows a cross-sectional view of the tubing of the prior art as itwent through the irradiation zone.

FIG. 4 shows a cross-sectional view of the tubing of this invention asit is flattened on the roller of the irradiation zone.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. I, the overall processis illustrated to produce a bilaterally oriented film. The relativelythick (30 mils) tubing 20 coming from the extruder is cooled, e.g., in awater bath or water spray (not shown), and conveyed by the nip rollersto the radiation cubicle. In this cubicle, an electron beam gun l0,e.g.,, of the type disclosed by the US. Pat. to Robinson No.

2,602,751, is arranged so that several passes of the tubing 20 areirradiated and ionized asthey are flattened on the flat primary rollers12 mounted directly under the electron beam gun It). This area thusconstitutes a flattening zone followed by an irradiation zone. Thetubing 20 is passed under the gun 10 a plurality of times. The number ofpasses is always an even number so each side of the tubing is exposed tothe electron beam the same number of times. It has been found inpractice when using an electron beam gun operation at 1 million voltswith a power output of 4,800 watts a dosage of between and 1.5 megarepsis obtained for each pass. The voltage can vary from 100 kv. to 10million volts in actual practice and the dosage range can vary from A to20 megareps, desirably between about 3 and about 8 megareps. Whenoperating within the desired range, it has been found that six passesunder the gun 10 achieved the desired results. Obviously, the number ofpasses can be varied by those skilled in the art to suit the availableequipment or the dosage per pass which desired. The tubing 20 is guidedby guide rollers 14 to the reversing wheels 16 and back to the gun 10 aplurality of times such as 3 or 5- times.

After passing back out of the cubicle, the now irradiated tubing 20 isheated in an oven and blown up over a trapped gas bubble in a mannerwell known to the art. At the same time the tubing is given this lateralstretch, the nip rolls 24 are giving it a longitudinal stretch due tothe fact that they are mov ing 3 times faster than the nip rolls 22mounted over the oven. Thus, the tubing 20 is given a bi-lateralorientation by the time it passes rollers 24,

In FIG. 2, an expanded view of the radiation conveyor is predeterminedvertical distance apart so that when the tubing shown. For clarity ofillustration, the details of the conveyor have been expanded along theaxis of the shaft 13 on which the flat rolls 12 are mounted. In actualpractice, the rolls 12 are mounted closely together directly under thegun 10. The guide rollers 14 are mounted below and on either side of theshaft 13. The inverting wheels 16 are independently mounted below theguide rolls 14 on shafts or axes 17 which are not parallel to the axesof the guide rollers. These shafts l7 diverge horizontally at an acuteangle to the centerline of the guide rolls 14 so that the tubing isreceived from one flat roller 12, reversed, and diverted slightly sothat it will feed on to the next adjacent flat roller. The lastreversing wheel 18 feeds the irradiated tubing from the last flat roll12 up and out of the eubicle.

It is thus seen that the rollers and wheels of the irradiation conveyorare not powered but are driven by the friction of the tubing passingover them.

The plastic tubing used in this process and apparatus can be made ofpolyethylene and polypropylene; copolymers of ethylene with propylene,isobutylene, amylene, acetylene, butadiene, and butene-l and butene-2;and terpolymers of ethylene, propylene and vinyl esters such as theacetate, propionate, butyrate.

During production runs using the processes and apparatus of the priorart, US. Pat. Nos, 3,022,543 and 2,9l4,450, it was found that thethickness of the finished film varied from 27.6 percent over the nominalthickness of 1.52 mils to 25 percent under the nominal thickness. Whenthe present invention was used these figures were reduced to 6.9 percentover and percent under the nominal thickness. This is a varia tion ofabout :8 percent which is well within the :10 percent variation which isacceptable to the food packaging industry.

lclaim:

1. Apparatus for the continuous production of uniformly irradiatedplastic tubing having a wall thickness of less than about 60 mils whichcomprises the combination of a. high energy electron irradiation means,

b. a plurality of primary roller means having a flat outer surfacemounted closely adjacent said irradiation means so as to receive theirradiation therefrom,

c. means for passing said tubing over said primary roller means and fortensioning said tubing passed over said primary roller means wherebysaid tubing is flattened by said primary roller means, and

d. a plurality of secondary roller means mounted on both sides of saidprimary roller means such that said tubing is inverted and diverted backto said primary roller means a plurality of times.

2. Apparatus for the continuous production of uniformly irradiatedplastic tubing having a wall thickness of less than about 60 mils whichcomprises the combination of a. a high energy electron beam generator,

b. a plurality of flat rollers mounted closely adjacent to saidgenerator so as to receive substantially all the generated electrons,

c. means for passing said tubing over said flat rollers and fortensioning said tubing passed over said flat rollers whereby said tubingis flattened by said flat rollers, and

d. a plurality of inverting wheels mounted on both sides of said flatrollers such that said tubing is inverted and diverted back to said flatrollers a plurality of times.

3. Apparatus for the continuous production of uniformly irradiatedplastic tubing having a wall thickness of less than about 60 mils whichcomprises the combination of a. a high energy electron beam generatormounted in a radiation shielding cubicle,

b. a plurality of flat rollers coaxially mounted directly under saidelectron generator,

c. a pair of guide rollers mounted on both sides of said flat rollersand parallel to said flat rollers,

d. means for passing said tubing over said flat rollers and fortensioning said tubing passed over said flat rollers whereby said tubingis flattened by said flat rollers, and e. a plura ity of invertingwheels mounted directly under said guide rollers on individual axeswhich diverge horizontally from the axes of said guide rollers such thatsaid tubing is inverted and diverted back to said flat rollers aplurality of times. 4. Apparatus as set forth in claim 3 in which sixflat rollers are used in conjunction with six inverting wheels.

1. Apparatus for the continuous production of uniformly irradiatedplastic tubing having a wall thickness of less than about 60 mils whichcomprises the combination of a. high energy electron irradiation means,b. a plurality of primary roller means having a flat outer surfacemounted closely adjacent said irradiation means so as to receive theirradiation therefrom, c. means for passing said tubing over saidprimary roller means and for tensioning said tubing passed over saidprimary roller means whereby said tubing is flattened by said primaryroller means, and d. a plurality of secondary roller means mounted onboth sides of said primary roller means such that said tubing isinverted and diverted back to said primary roller means a plurality oftimes.
 2. Apparatus for the continuous production of uniformlyirradiated plastic tubing having a wall thickness of less than about 60mils which comprises the combination of a. a high energy electron beamgenerator, b. a plurality of flat rollers mounted closely adjacent tosaid generator so as to receive substantially all the generatedelectrons, c. means for passing said tubing over said flat rollers andfor tensioning said tubing passed over said flat rollers whereby saidtubing is flattened by said flat rollers, and d. a plurality ofinverting wheels mounted on both sides of said flat rollers such thatsaid tubing is inverted and diverted back to said flat rollers aplurality of times.
 3. Apparatus for the continuous production ofuniformly irradiated plastic tubing having a wall thickness of less thanabout 60 mils which comprises the combination of a. a high energyelectron beam generator mounted in a radiation shielding cubicle, b. aplurality of flat rollers coaxially mounted directly under said electrongenerator, c. a pair of guide rollers mounted on both sides of said flatrollers and parallel to said flat rollers, d. means for passing saidtubing over said flat rollers and for tensioning said tubing passed oversaid flat rollers whereby said tubing is flattened by said flat rollers,and e. a plurality of inverting wheels mounted directly under said guiderollers on individual axes which diverge horizontally from the axes ofsaid guide rollers such that said tubing is inverted and diverted backto said flat rollers a plurality of times.
 4. Apparatus as set forth inclaim 3 in which six flat rollers are used in conjunction with sixinverting wheels.